<!DOCTYPE html>
<style>
body { text-transform: uppercase; width: 200px; height: 200px; background-color: white; }
#a, #b { font-size: 24px; }
</style>
<div id=a><div id=b><span>x</span><span>y</span></div></div>
<script>
document.body.offsetTop;
var a = document.getElementById("a");
var b = document.getElementById("b");
a.style.fontSize = "24px";
b.style.fontSize = "24px";
document.body.offsetTop;
b.style.fontSize = "36px";
document.body.offsetTop;
</script>
